This invention relates to a photoconductive material. More particularly, it relates to a photoconductive material which exhibits a high response speed and can be easily controlled in sensitivity not only to long wavelength light but also to short wavelength light.
As the photoconductive material which absorbs the energy of the electromagnetic radiation such as ultraviolet rays, visible rays, infrared rays and X-rays to produce carriers for electric charge and increase the electroconductivity, there are known inorganic photoconductive materials such as Se, CdS, ZnO and As.sub.2 S.sub.3 as well as organic photoconductive materials such as poly-N-vinylcarbazole, trinitrofluorenone, phthalocyanine compounds and triphenyl-amine-polycarbonate. While these conventional inorganic or organic photoconductive materials are utilized in various fields depending upon their photoconductive characteristics, they have more or less some certain drawbacks; hence it is always necessary for their practical use to make some adjustments for overcoming those drawbacks. Thus, they are not satisfactory for overall purposes.
In general, organic photoconductive materials can be readily molded in a sheet or film form and easily controlled in sensitivity to the wavelength of light. Since, however, the mobility of the carrier for electric charge is small, their application in the field requiring a high speed response is restricted. On the other hand, inorganic photoconductive materials can show a high mobility of the carrier for electric charge. But, the control of their sensitivity to the wavelength of light is difficult. Even if successful in controlling the sensitivity, other characteristic properties such as the mobility of the carrier, the lifetime of the carrier and the proportion of the photoconductivity to the dark conductivity are lowered. In order to overcome the above drawbacks inherent to organic or inorganic photoconductive materials, attempts have been made to design photoconductive materials of separation-of-function type which are constituted with organic photoconductive materials and inorganic photoconductive materials in combination. However, no satisfactory one has been obtained yet.
With respect to photosensitivity, response speed, durability, stability in molding technique, etc., a-Se (the prefix "a-" meaning "amorphous"), Cd-S, Cd-Se, a-Se-As-Te, etc. are favorable photoconductive materials. In particular, a-Se has been practically utilized as the photoconductive material for copying machines over a long period of time. This a-Se material has a large dark resistance (i.e. 10.sup.13 to 10.sup.15 ohm.cm), and when irradiated with light, the resistivity is greatly decreased. With a-Se, a photoconductive film of stabilized quality can be readily prepared by vacuum evaporation. The trap level present in the bulk of a-Se is low, and the mobility of the hole as the major carrier is about 0.2 cm.sup.2 /V.sec. so that it can correspond to a high speed response. As well known, however, a-Se has a highly sensitive area near the wavelength of 470 nm and shows little sensitivity to the wavelength of 600 nm or more. This is because the generation step of the photo carrier is controlled by the geminate recombination so that the generation efficiency (.eta.) of the photo carrier rapidly decreases against light having a long wavelength; the application field of a-Se is thus restricted. Also, a-Se crystallizes when irradiated with strong light or heated, and its photoconductive characteristics are markedly deteriorated. For prevention of these defects, attempts have been made to incorporate As, Te, etc. therein. However, the photoconductive material of Se containing Te and As (i.e. a-Se-As-Te) produces the increase of light fatigue as well as the deterioration of response characteristics.
Compound semi-conductors such as CdS and CdSe have a high photosensitivity and are excellent in heat stability. However, the preparation of uniform films with them is difficult, and their molding can be made only by disadvantageous procedures comprising sintering of fine particles or mixing with resinous binders.
Accordingly, the appearance of a photoconductive material which can be readily prepared in the form of thin film, has a great dark resistance, exerts good photoconductive characteristics on light irradiation and is excellent in sensitivity to long wavelength light has been highly demanded. Such photoconductive material would be widely applicable to various fields and can be utilized, for instance, in such laser printers, line sensors, etc. as using semi-conductor laser, which require a high speed response. Since, however, the conductivity (.sigma.d) at dark is elevated with the increase of the energy gap (Eg) between the valency electron zone and the conductor, it will be hardly possible to achieve the increase of the sensitivity to long wavelength light and the decrease of the dark current simultaneously. This invention is directed to the solution of this inconsistent technical problem.